Para-xylene is typically recovered from a predominantly C8 aromatic hydrocarbon fraction derived from various sources, such as catalytic reforming, by liquid-liquid extraction and/or fractional distillation. The C8 aromatic hydrocarbon fraction is normally subjected to xylene isomerization to increase the concentration of the desired para-isomer and the para-xylene is then separated from the resultant product stream, usually containing all three xylene isomers, by either crystallization or adsorptive separation or a combination of these two techniques. Adsorptive separation is the newer technique and has captured the great majority of the market share of newly constructed plants for the production of para-xylene.
Essentially all of these adsorptive separation units use a simulated countercurrent movement of the adsorbent and the xylene containing feed stream. This simulation is performed using established commercial technology wherein the adsorbent is held in place in one or more cylindrical adsorbent chambers and the positions at which the streams involved in the process enter and leave the chambers are slowly shifted along the length of the beds. Normally there are at least four streams (feed, desorbent, extract and raffinate) employed in this procedure and the location at which the feed and desorbent streams enter the chamber and the extract and raffinate streams leave the chamber are simultaneously shifted in the same direction at set intervals. Each shift in location of these transfer points delivers or removes liquid from a different bed within the chamber. This shifting could be performed using a dedicated line for each stream at the entrance to each bed. However, this would greatly increase the cost of the process and therefore the lines are reused and each line carries one of the four process streams at some point in the cycle.
The general technique employed in the performance of a simulated moving bed adsorptive separation is well described in the open literature. For instance a general description directed to the recovery of para-xylene was presented at page 70 of the September 1970 edition of Chemical Engineering Progress (Vol. 66, No 9). A generalized description of the process with an emphasis on mathematical modeling was given at the International Conference on “Fundamentals of Adsorption”, Schloss Elmau, Upper Bavaria, Germany on May 6-11, 1983 by D. B. Broughton and S. A. Gembicki. U.S. Pat. No. 4,029,717 issued to F. J. Healy et al. describes a simulated moving bed adsorptive separation process for the recovery of para-xylene from a mixture of xylene isomers. Numerous other available references describe many of the mechanical parts of a simulated moving bed system, including rotary valves for distributing various liquid flows, the internals of the adsorbent chambers and control systems.
The prior art recognizes that the presence of residual compounds in the transfer lines can have detrimental effects on the simulated moving bed process. For example, U.S. Pat. No. 3,201,491 and International Patent Publication WO 95/07740 both address the flushing of the line used to deliver the feed stream to the adsorbent chamber as a means to increase the purity of the recovered extract or sorbate component. This step avoids contamination of the extract stream with raffinate components of the feed remaining in this line when it is subsequently used to withdraw the extract stream from the adsorbent chamber. Both references employ a desorbent rich steam to flush the contents of this line back into the adsorbent chamber.
In addition, U.S. Pat. No. 5,912,395 discloses that the capacity of a simulated moving bed adsorptive separation process can be increased by flushing the contents of the transfer line just previously used to remove the raffinate stream from the adsorbent chamber back into the adsorbent chamber. This flushing step is performed immediately upstream of the point of raffinate withdrawal and eliminates the passage of raffinate material into the adsorbent chamber when the transfer line is subsequently used to charge the feed stream to the adsorbent chamber. The flushing liquid is preferably the feed stream to the process.
In contrast, U.S. Pat. No. 7,208,651 discloses a simulated moving bed adsorptive separation process in which the contents of the transfer line previously used to remove the raffinate stream from the adsorbent chamber is flushed away from the adsorbent chamber into the raffinate column used to separate desorbent from raffinate product. The flushing liquid is a stream from the adsorbent chamber at an intermediate point between the feed entry point and raffinate withdrawal. This flushing step is intended to eliminate the passage of raffinate material into the adsorbent chamber in the transfer-line flush period or when the process conduit is subsequently used to charge the feed stream to the adsorbent chamber.
According to the present invention, a simulated moving bed adsorptive separation process is provided in which the contents of the transfer line just previously used to supply the desorbent stream to the adsorbent chamber is flushed into the adsorbent chamber. This flushing step is performed at an intermediate point along the column between the desorbent entry point and raffinate withdrawal point. The flushing liquid is conveniently the raffinate flush stream. This flushing step maximizes utilization of the desorbent and, by reducing the amount of unused desorbent flowing to the raffinate tower, reduces the energy requirements of the raffinate tower.